The treatment of pain symptoms, e.g., post-operative pain or pain resulting from disease or injury, frequently entails the subcutaneous and/or intravenous (“IV”) infusion of a liquid analgesic and/or anesthetic drug into the patient, e.g., by one or more hypodermic injections thereof. When pain is more chronic, it may be preferable to catheterize the patient intravenously, e.g., with a hypodermic needle, and infuse the drug through the catheter continuously at a low, or “basal,” rate of flow using, e.g., an “IV drip” or an electromechanical pump having an adjustably low flow rate.
Many patients who exhibit chronic pain symptoms also experience periodic episodes in which the pain level is perceived as much more acute, indicating a need for a temporarily greater infusion rate of the drug. One solution is to adjust the flow rate of the infusion apparatus. However, this requires the presence and intervention of a trained health care professional, as the patient typically lacks the training, skill and/or physical ability to make such an adjustment of the infusion device.
There have been a number of proposals for a “Patient Controlled Analgesic,” or “PCA,” drug administration device that would enable a patient to self-administer a drug intravenously without intervention by a health care professional at, e.g., the onset of an acute pain episode, that is not only simple and effortless to operate, but is also failsafe in use, i.e., one that precludes the possibility of a self-administered drug overdose. Examples of such PCA devices can be found in U.S. Pat. No. 5,084,021 to B. Baldwin; U.S. Pat. No. 5,891,102 to K. Hiejima et al.; and U.S. Pat. No. 6,213,981 to K. Hiejima et al.
These devices all have in common the provision of a reciprocating pump in which the patient manually effects a “compression” stroke of the pump by depressing a plunger of the pump, thereby expressing a measured bolus of a liquid drug to the patient intravenously, after which a compression spring and/or a pressurized source of the drug returns the plunger to its initial position, thereby effecting a refill, or “intake,” stroke of the pump. The rate at which the pump refills, and hence, the rate at which the patient may self-administer the drug, is limited by a flow restrictor placed at the inlet of the pump. A reverse flow of fluids from the patient to the pump may be effected by a check valve disposed at the outlet of the pump.
These PCA devices afford only a partial solution to the problem of a patient controlled drug administration device because they present certain drawbacks. For example, some require that the patient continuously exert a force on the plunger throughout the compression stroke of the pump, which may take several seconds or even minutes to complete, and some patients may not be physically capable of such a prolonged exertion. Others require that the patient push a first button down on the pump to effect the compression stroke, then push a second button on the pump to initiate the intake stroke, which may also be of prolonged duration, after the compression stroke is complete, which requires that the patient monitor the position of the plunger to know when to push the second button. An additional drawback shared by all is that they require an extended period of time, and require a careful manipulation of the device by a health care professional, to “prime” the device before use, i.e., to replace any air in the device with the liquid drug, since the administration of any air bubbles to the patient could form a dangerous embolism in the patient.
Another PCA device is described in U.S. Pat. No. 6,936,035 to Rake et al. That device can administer either or both of a continuous and a bolus infusion of a liquid drug to a patient, in which the bolus doses can be safely self-administered by the patient by quickly depressing a single button, to effect the compression stroke of the pump, and which thereafter automatically initiates the intake stroke of the pump when the compression stroke is completed, and further, one which can be rapidly primed for use without skilled manipulation of the device. However, there is still a need for a PCA device that can avoid the potential for bolus refill during bolus delivery. Should it happen, such a condition may allow delivery of more liquid drug to a patient over a defined period of time than the predetermined volume of successive boluses. There is also a need for a PCA device that can avoid the potential for liquid drug delivery through the pump after bolus delivery but before an intake stroke of the pump occurs. This can be particularly undesirable if the pump malfunctions and the pump activation button becomes stuck in its compression stroke. Should it happen, such a stuck condition may allow continued delivery of the liquid drug through the pump after bolus delivery but before an intake stroke of the pump occurs.